Planar (and other types of) antenna systems often require the antenna to fit into ever-shrinking available spaces while maintaining key performance characteristics, such as high ohmic efficiency and broad band operation. To achieve the desired performance, a hybrid combination of parallel-plate and waveguide designs are often used as propagation media due to their superior bandwidth and ohmic efficiency characteristics. FIG. 1 illustrates a generic parallel-plate transmission line 10 and waveguide transmission line 12. The parallel-plate transmission line is defined by an upper conductive plate 10a and a lower conductive plate 10b arranged parallel to the upper conductive plate. The waveguide transmission line 12 is defined by an enclosed region having two narrow walls 12a arranged opposite one another, and two wide (broad) walls 12b arranged opposite one another, the narrow and wide (broad) walls joined together to define a volume.
The waveguide portion is usually deployed (arrayed) in a corporate feed, traveling-wave feed, standing-wave feed, or other structure where multiple outputs are coupled to a common parallel-plate section. To support the hybrid combination of transmission lines and support efficient performance over a wide frequency bandwidth (including the effects of higher-order modes associated with the mutual coupling between adjacent/proximal waveguide outputs), a coupling transition is provided between the two media.
A typical method for transitioning from waveguide to parallel plate involves the use of a tapered horn, but such devices are typically quite large and difficult to package. Another method for transitioning from parallel plate to a waveguide corporate feed network in which multiple waveguide outputs have been generated using E-plane power dividers (Tees) involves the attachment of multiple separate (flanged) waveguide twist components. This approach has its drawbacks in terms of packaging size, manufacturing complexity, and cost, and these drawbacks become even more pronounced as operating frequencies are increased.
Referring to FIG. 2, illustrated is a conventional structure 20 for transitioning from a waveguide transmission line 12 to a parallel-plate transmission line 10, the respective transmission lines being located on the same plane and E-fields orthogonal to one another. The waveguide transmission line 12 includes a waveguide feed input 22 for receiving an RF signal, and a waveguide corporate feed 24 coupled to the feed input 22. The waveguide corporate feed 24 includes a plurality of feed paths branching off from the waveguide feed input 22. A flange 26 or other mounting means is coupled to respective output ports of the waveguide corporate feed 24 to facilitate attachment of the waveguide transmission line 12 to another structure. The flange 26 includes a plurality of openings (not shown in FIG. 2), each opening corresponding to an output port of the corporate feed structure.
A parallel plate transmission line 10 includes an upper plate 10a and a lower plate 10b generally parallel to the upper plate, the upper and lower plates defining the transmission line. A flange 30 or other mounting means is connected to the upper and lower plates to facilitate attachment of the parallel plate structure to another structure. The flange 30 includes an opening (not shown in FIG. 2) corresponding to a separation (gap) between the upper and lower plates 10a, 10b. 
Coupling the waveguide transmission line 12 to the parallel plate transmission line 10 are a plurality of waveguide twist structures 40. Each waveguide twist structure 40 may include input and output flanges 46, 48 or other mounting means for coupling the waveguide twist structure 40 to the waveguide transmission line flange 26 and the parallel plate transmission line 10 flange 30.
FIG. 3 shows another conventional method for transitioning waveguide to parallel-plate when such transmission media are located in the same plane and their E-fields are orthogonal to one another. The device 50 shown in FIG. 3 employs a tapered horn configuration having an input port 52 for connection to a waveguide transmission line (not shown in FIG. 3), and an output port 54 for connection to a parallel plate transmission line 10. A region 56 (tapered horn) between the input port 52 and the output port 54 is tapered to correspond to the respective ports 52, 54. A separate waveguide twist 57 is attached at the port 52 to provide the desired orthogonality of the E-fields at the port 58 as achieved in FIG. 2.
While the approaches shown in FIGS. 2 and 3 achieve the desired result, in a practical case where multiple waveguides or a tapered horn are used to feed a large parallel-plate region, feeding such a structure may be a challenge in the available space (which is usually confined to the total area provided by the product). Further, multiple Twist Waveguide structures add undesired depth and length to the parallel plate and waveguide ensemble.
The tapered horn approach typically requires too much product area to effectively package, and the separately attached twist component requires a significant amount of product thickness to allow for fitting the requisite waveguide flange. In addition, tapered horns can suffer from dimensional variation over the large, unsupported area of the device. Such dimensional variation can result in phase errors and degraded performance. In some cases tapered horns can be folded over to make the device more compact, mitigating the effect of dimensional variation, but this also has the negative effect of making the device thicker and more expensive to fabricate.